This invention relates to a heat resistant austenitic cast steel with improved mechanical properties such as mechanical strength under high temperatures.
Austenitic steel has a high corrosion resistance and, thus, is widely used as a material for articles used under corrosive conditions. Also, the mechanical properties of austenitic steel are effected less by temperature than those of ferritic steel, making it possible to increase the upper limit of temperature to which austenitic steel can be exposed. Therefore, its application will be broader than ferritic steel.
However, the mechanical strength of austenitic steel is lower than that of ferritic steel. Thus, in order to use the austenitic steel specified by JIS SUS 304 or 316 under high temperatures, it is necessary to reinforce the austenitic steel article or part by increasing the thickness thereof. If the thickness is increased, it is naturally difficult to transport or install the article or part, particularly where the article or part is large. Also, a large temperature gradient is brought about in the thickness direction of the article in the heating step of the article. If heating-cooling treatment is repeatedly applied, thermal fatigue of the article is promoted. Thus, in order to actually increase the upper limit of temperature under which the austenitic steel can be used, it is necessary to improve the mechanical properties of the steel under room temperature and high temperatures.
On the other hand, it is difficult to apply hot forging and cold working to a large article of complex shape such as a turbine casing. Thus, such a large article is produced in many cases by casting. However, the mechanical strength of castings is lower than that of a hot forged article or cold worked material, with the result that the castings should be made thicker. Also, segregation tends to occur in the castings because forging, pressing or the like is not applied to the casting material, resulting in a restriction in the amounts of additional elements that can be used with the casting material. It is also impossible to increase the mechanical strength of the castings by the treatment to diminish the grains.
When it comes to nickel-based alloys, the mechanical strength is increased by precipitating .gamma.'-phase, such as Ni.sub.3 Al, in the alloys. However, the .gamma.'-phase precipitation results in the reduction in the elongation and reduction of area of the material, and requires complex heat treatments. Particularly where the casting defect remains as it is in the castings, the precipitation is changed in the welding step for repairing the casting defect so that the mechanical properties of the material deteriorate. Under the circumstances, it is not practical to increase the mechanical strength of the castings by the .gamma.'-phase precipitation.
In a thermal power plant using coal or petroleum as the fuel, it is necessary to further heat and pressurize the steam to, for example, 1100.degree. F. and 352 atms. for improving the thermal efficiency. It was customary to use a martensite cast steel such as Cr-Mo-V steel in the turbine of such a thermal power plant. However, since the martensitic cast steel is low in its mechanical strength under high temperatures, it has been attempted to use austenitic cast steel, which is superior to the martensitic cast steel in mechanical strength under high temperatures, for forming such a turbine. Particularly, the turbine casing receives a load of high pressure steam and, thus, requires an improvement in the mechanical strength of the material of which it is formed.
Also, the operating conditions of chemical plants and boilers are becoming more server, leading to a strong demand for the development of a material having a mechanical strength high enough for the material to be used under extremely high temperatures and pressures.
Austenitic cast steel exhibits relatively satisfactory high temperature characteristics, compared with the other materials. However, further improvements are required in its high temperature characteristics such as mechanical stress, proof stress, creep rupture strength, elongation and reduction of area, to enable the austenitic cast steel to be used in the actual apparatus satisfactorily.